None, but this doesn't help your case one bit. In fact it's rather damaging to the UFO conspiracy theory.
World govts do not agree on much. This is why there is war, why there was NATO and the Warsaw pact, why some joined neither, why some are pro-nuclear, and others anti.
Why then, would they all agree to cover up the existence of UFOs/ETIs?
During the cold war for example, it would have been an incredible propaganda advantage for one side to state they had captured alien technology and were reverse engineering it to create a super weapon. The other side would have been desperate to capture their own, and been wasting time and resources as there was actually none to be had. Disinformation, you understand this, right?
But anyway, there's a problem with such a conspiracy even if world govts did agree,... since when did they have good communications,... good enough to make such an agreement? The Victorians laid telegraph cables under the sea, and across land, to enable Morse code transmissions to the colonies, and that was just the bits us Brits owned at the time. We are talking ~150 years ago, to get a Morse message across the Atlantic. Transatlantic radio commenced ~100 years ago. These communications methods were 'hub and spoke', central hubs had messages relayed to them, which were transmitted, and relayed again, so there were various steps, not all using the technology, so some comms would end up being post. So say, 120 years ago an alien craft crash lands in America,... the local Marshall saddles up and goes to see his boss, and rides, his boss may have access to a Telegraph, so sends a message to the State governor, who relays it to Washington, who relays it to other states to warn them, and other countries. How long does this take? Locals will pass the story by word of mouth, photography was possible back then newspapers printed pictures, and there wasn't really any way to stifle such publications, as control was devolved. It would take weeks to get this organised.
So then later on, communications becomes radio, or telephone, there are better roads, then the govt have aircraft, and can communicate, and travel much faster. Say post WWII, with jet aircraft. ~60 years ago, they perhaps had the capability of stifling such activity. But then, the public started getting better communications too, first cellphones, then the Internet, and then better yet, cellphones with cameras, and the Internet! I can take a picture, and upload it in seconds,... how could the Govt now stifle the public from disclosing what they see? They can't. So the window for conspiracy was short, even if all world Govts agreed on this one thing, which, given differences in religion, and political ideologies, that seems farcical.
You need to propose a sound mechanism via which such a worldwide conspiracy could work. Oh, and don't try the 'One World Govt' angle, if that were true, we'd all be wearing little uniforms, and working in factories and coal mines. Progress is slow when large numbers of people are involved.
